The development of highly absorbent members for use as disposable diapers, adult incontinence pads and briefs, and catamenial products such as sanitary napkins, are the subject of substantial commercial interest. A highly desired characteristic for such products is thinness. For example, thinner diapers are less bulky to wear, fit better under clothing, and are less noticeable. They are also more compact in the package, making the diapers easier for the consumer to carry and store. Compactness in packaging also results in reduced distribution costs for the manufacturer and distributor, including less shelf space required in the store per diaper unit.
The ability to provide thinner absorbent articles such as diapers has been contingent on the ability to develop relatively thin absorbent cores or structures that can acquire and store large quantities of discharged body fluids, in particular urine. In this regard, the use of certain absorbent polymers often referred to as "hydrogels," "superabsorbents" or "hydrocolloid" material has been particularly important. See, for example, U.S. Pat. No. 3,699,103 (Harper et al), issued Jun. 13, 1972, and U.S. Pat. No. 3,770,731 (Harmon), issued Jun. 20, 1972, that disclose the use of such absorbent polymers (hereafter "hydrogel-forming absorbent polymers") in absorbent articles. Indeed, the development of thinner diapers has been the direct consequence of thinner absorbent cores that take advantage of the ability of these hydrogel-forming absorbent polymers to absorb large quantities of discharged body fluids, typically when used in combination with a fibrous matrix. See, for example, U.S. Pat. No. 4,673,402 (Weisman et al), issued Jun. 16, 1987 and U.S. Pat. No. 4,935,022 (Lash et al), issued Jun. 19, 1990, that disclose dual-layer core structures comprising a fibrous matrix and hydrogel-forming absorbent polymers useful in fashioning thin, compact, nonbulky diapers.
Prior to the use of these hydrogel-forming absorbent polymers, it was general practice to form absorbent structures, such as those suitable for use in infant diapers, entirely from wood pulp fluff. Given the relatively low amount of fluid absorbed by wood pulp fluff on a gram of fluid absorbed per gram of wood pulp fluff, it was necessary to employ relatively large quantities of wood pulp fluff, thus necessitating the use of relatively bulky, thick absorbent structures. The introduction of these hydrogel-forming absorbent polymers into such structures has allowed the use of less wood pulp fluff. These hydrogel-forming absorbent polymers are superior to fluff in their ability to absorb large volumes of aqueous body fluids, such as urine (i.e., at least about 15 g/g), thus making smaller, thinner absorbent structures feasible.
These hydrogel-forming absorbent polymers are often made by initially polymerizing unsaturated carboxylic acids or derivatives thereof, such as acrylic acid, alkali metal (e.g., sodium and/or potassium) or ammonium salts of acrylic acid, alkyl acrylates, and the like. These polymers are rendered water-insoluble, yet water-swellable, by slightly cross-linking the carboxyl group-containing polymer chains with conventional di- or poly-functional monomer materials, such as N,N'-methylenebisacrylamide, trimethylol propane triacrylate or triallyl amine. These slightly crosslinked absorbent polymers still comprise a multiplicity of anionic (charged) carboxyl groups attached to the polymer backbone. It is these charged carboxy groups that enable the polymer to absorb body fluids as the result of osmotic forces, thus forming hydrogels.
The degree of cross-linking determines not only the water-insolubility of these hydrogel-forming absorbent polymers, but is also an important factor in establishing two other characteristics of these polymers: their absorbent capacity and gel strength. Absorbent capacity or "gel volume" is a measure of the amount of water or body fluid that a given amount of hydrogel-forming polymer will absorb. Gel strength relates to the tendency of the hydrogel formed from these polymers to deform or "flow" under an applied stress. Hydrogel-forming polymers useful as absorbents in absorbent structures and articles such as disposable diapers need to have adequately high gel volume, as well as adequately high gel strength. Gel volume needs to be sufficiently high to enable the hydrogel-forming polymer to absorb significant mounts of the aqueous body fluids encountered during use of the absorbent article. Gel strength needs to be such that the hydrogel formed does not deform and fill to an unacceptable degree the capillary void spaces in the absorbent structure or article, thereby inhibiting the absorbent capacity of the structure/article, as well as the fluid distribution throughout the structure/article. See, for example, U.S. Pat. No. 4,654,039 (Brandt et al), issued Mar. 31, 1987 (reissued Apr. 19, 1988 as U.S. Re. Pat. No. 32,649) and U.S. Pat. No. 4,834,735 (Alemany et al), issued May 30, 1989.
Prior absorbent structures have generally comprised relatively low amounts (e.g., less than about 50% by weight) of these hydrogel-forming absorbent polymers. See, for example, U.S. Pat. No. 4,834,735 (Alemany et al), issued May 30, 1989 (preferably from about 9 to about 50% hydrogel-forming absorbent polymer in the fibrous matrix). There are several reasons for this. The hydrogel-forming absorbent polymers employed in prior absorbent structures have generally not had an absorption rate that would allow them to quickly absorb body fluids, especially in "gush" situations. This has necessitated the inclusion of fibers, typically wood pulp fibers, to serve as temporary reservoirs to hold the discharged fluids until absorbed by the hydrogel-forming absorbent polymer.
More importantly, many of the known hydrogel-forming absorbent polymers exhibited gel blocking. "Gel blocking" occurs when particles of the hydrogel-forming absorbent polymer are wetted and the particles swell so as to inhibit fluid transmission to other regions of the absorbent structure. Wetting of these other regions of the absorbent member therefore takes place via a very slow diffusion process. In practical terms, this means acquisition of fluids by the absorbent structure is much slower than the rate at which fluids are discharged, especially in gush situations. Leakage from the absorbent article can take place well before the particles of hydrogel-forming absorbent polymer in the absorbent member are fully saturated or before the fluid can diffuse or wick past the "blocking" particles into the rest of the absorbent member. Gel blocking can be a particularly acute problem if the particles of hydrogel-forming absorbent polymer do not have adequate gel strength and deform or spread under stress once the particles swell with absorbed fluid. See U.S. Pat. No. 4,834,735 (Alemany et al), issued May 30, 1989.
This gel blocking phenomena has typically necessitated the use of a fibrous matrix in which are dispersed the particles of hydrogel-forming absorbent polymer. This fibrous matrix keeps the particles of hydrogel-forming absorbent polymer separated from one another. This fibrous matrix also provides a capillary structure that allows fluid to reach the hydrogel-forming absorbent polymer located in regions remote from the initial fluid discharge point. See U.S. Pat. No. 4,834,735 (Alemany et al), issued May 30, 1989. However, dispersing the hydrogel-forming absorbent polymer in a fibrous matrix at relatively low concentrations in order to minimize or avoid gel blocking can lower the overall fluid storage capacity of thinner absorbent structures. Using lower concentrations of these hydrogel-forming absorbent polymers limits somewhat the real advantage of these materials, namely their ability to absorb and retain large quantities of body fluids per given volume.
Besides increasing gel strength, other physical and chemical characteristics of these hydrogel-forming absorbent polymers have been manipulated to decrease gel blocking. One characteristic is the particle size, and especially the particle size distribution, of the hydrogel-forming absorbent polymer used in the fibrous matrix. For example, particles of hydrogel-forming absorbent polymer having a particle size distribution such that the particles have a mass median particle size greater than or equal to about 400 microns have been mixed with hydrophilic fibrous materials to minimize gel blocking and to help maintain an open capillary structure within the absorbent structure so as to enhance planar transport of fluids away from the area of initial discharge to the rest of the absorbent structure. In addition, the particle size distribution of the hydrogel-forming absorbent polymer can be controlled to improve absorbent capacity and efficiency of the particles employed in the absorbent structure. See U.S. Pat. No. 5,047,023 (Berg), issued Sep. 10, 1991. However, even adjusting the particle size distribution does not, by itself, lead to absorbent structures that can have relatively high concentrations of these hydrogel-forming absorbent polymers. See U.S. Pat. No. 5,047,023, supra (optimum fiber to particle ratio on cost/performance basis is from about 75:25 to about 90:10).
Another characteristic of these hydrogel-forming absorbent polymers that has been looked at is the level of extractables present in the polymer itself. See U.S. Pat. No. 4,654,039 (Brandt et al), issued Mar. 31, 1987 (reissued Apr. 19, 1988 as U.S. Re. Pat. No. 32,649). Many of these hydrogel-forming absorbent polymers contain significant levels of extractable polymer material. This extractable polymer material can be leached out from the resultant hydrogel by body fluids (e.g., urine) during the time period such body fluids remain in contact with the hydrogel-forming absorbent polymer. It is believed such polymer material extracted by body fluid in this manner can alter both the chemical and physical characteristics of the body fluid to the extent that the fluid is more slowly absorbed and more poorly held by the hydrogel in the absorbent article.
Another characteristic that has been looked at to minimize gel blocking is to improve the capillary capability of these hydrogel-forming absorbent polymers. In particular, it has been suggested that particles of these hydrogel-forming absorbent polymers be formed into interparticle crosslinked aggregate macrostructures, typically in the form of sheets or strips. See U.S. Pat. No. 5,102,597 (Roe et al), issued Apr. 7, 1992; U.S. Pat. No. 5,124,188 (Roe et al), issued Jun. 23, 1992; and U.S. Pat. No. 5,149,344 (Lahrman et al), issued Sep. 22, 1992. Because the particulate nature of the absorbent polymer is retained, these macrostructures provide pores between adjacent particles that are interconnected such that the macrostructure is fluid permeable (i.e., has capillary transport channels). Due to the interparticle crosslink bonds formed between the particles, the resultant macrostructures also have improved structural integrity, increased fluid acquisition and distribution rates, and minimal gel blocking characteristics.
Yet another characteristic the art has known for some time as a measure of gel blocking is the Demand Wettability or Gravimetric Absorbence of these hydrogel-forming absorbent polymers. See, for example, U.S. Pat. No. 5,147,343 (Kellenberger), issued Sep. 15, 1992 and U.S. Pat. No. 5,149,335 (Kellenberger et al), issued Sep. 22, 1992 where these hydrogel-forming absorbent polymers are referred to as "superabsorbent materials" and where Demand Wettability/Gravimetric Absorbence is referred to as Absorbency Under Load (AUL). "AUL" is defined in these patents as the ability of the hydrogel-forming absorbent polymer to swell against an applied restraining force (see U.S. Pat. No. 5,147,343, supra, at Col. 2, lines 43-46). The "AUL value" is defined as the amount (in ml./g or g/g.) of 0.9% saline solution that is absorbed by the hydrogel-forming absorbent polymers while being subjected to a load of 21,000 dynes/cm.sup.2 (about 0.3 psi). The AUL value can be determined at 1 hour (see U.S. Pat. No. 5,147,343) or 5 minutes (see U.S. Pat. No. 5,149,335). Hydrogel-forming absorbent polymers are deemed to have desirable AUL properties if they absorb at least about 24 ml./g (preferably at least about 27 ml./g) of the saline solution after 1 hour (see U.S. Pat. No. 5,147,343) or at least about 15 g/g (preferably at least about 18 g/g) of the saline solution after 5 minutes.
AUL as defined in U.S. Pat. Nos. 5,147,343 and 5,149,335 may provide some indication of which hydrogel-forming absorbent polymers will avoid gel blocking in some instances. However, AUL is inadequate for determining which hydrogel-forming absorbent polymers will provide the absorbency properties necessary so that the concentration of these polymers in absorbent structures can be increased without significant gel blocking or some other undesirable effect. Indeed, certain of the hydrogel-forming absorbent polymers disclosed in U.S. Pat. Nos. 5,147,343 and 5,149,335 as having satisfactory AUL values will have inadequate permeability to be useful at high concentrations in absorbent members. In order to have a high AUL value, it is only necessary that the hydrogel layer formed have at least minimal permeability such that, under a confining pressure of 0.3 psi, gel blocking does not occur to any significant degree. The degree of permeability needed to simply avoid gel blocking is much less than that needed to provide good fluid transportation properties. Hydrogel-forming absorbent polymers that avoid gel blocking can still be greatly deficient in other fluid handling properties.
Another problem with using AUL values measured according to U.S. Pat. Nos. 5,147,343 and 5,149,335 is that they do not reflect all of the potential pressures that can be operative on the hydrogel-forming polymer in the absorbent structure. As noted above, AUL is measured in these patents at a pressure of about 0.3 psi. It is believed that a much higher confining pressure of about 0.7 psi more adequately reflects the full range of localized mechanical pressures (e.g., sitting, sleeping, squatting, taping, elastics, leg motions, other tension and torsional motions) on an absorbent structure. See U.S. Pat. No. 5,147,345 (Young et al), issued Sep. 15, 1992. Additionally, many of the absorbent structures that comprise these hydrogel-forming absorbent polymers can include other components, such as an acquisition layer that receives the initial discharge of body fluids. See, for example, U.S. Pat. No. 4,673,402 (Weisman et al), issued Jun. 16, 1987 and U.S. Pat. No. 4,935,022 (Lash et al), issued Jun. 19, 1990. This acquisition layer can comprise fibers, such as certain chemically stiffened fibers, that have a relatively high capillary suction. See, for example, U.S. Pat. No. 5,217,445 (Young et al), issued Jun. 8, 1993. To take into account these additional capillary pressures that could affect fluid acquisition by these hydrogel-forming absorbent polymers, it is more realistic to measure demand absorbency performance under a higher pressure, i.e., about 0.7 psi. This would take into better account not only the localized mechanical pressures exerted during use, but also the additional capillary pressures resulting from other components (e.g., acquisition layer) present in the absorbent structure.
For absorbent structures having relatively high concentrations of these hydrogel-forming absorbent polymers, other characteristics of these absorbent polymers have been evaluated. See, for example, European patent application 532,002 (Byedy et al), published Mar. 17, 1993, which identifies a characteristic called Deformation Under Load (DUL) as being important for absorbent composites having high concentrations of hydrogel-forming absorbent polymers. "DUL" is used in European patent application 532,002 to evaluate the ability of the hydrogel-forming absorbent polymer to maintain wicking channels after the absorbent polymer is swollen. See page 3, lines 9-10. DUL values are obtained by incompletely saturating the hydrogel-forming absorbent polymer with a fixed amount of synthetic urine, compressing the absorbent polymer under a light load (0.3 psi), and then measuring the deformation of the absorbent polymer under a heavier load (0.9 psi). See page 5, lines 37-40. Hydrogel-forming absorbent polymers having DUL values of about 0.6 mm or less (preferably about 0.5 mm or less, most preferably about 0.3 mm or less) are deemed to be desirable. See page 4, lines 1-3.
DUL as defined in European patent application 532,002 may provide some indication of the ability of hydrogel-forming absorbent polymer to maintain wicking channels after the absorbent polymer is swollen. However, it has been found that the openness or porosity of the hydrogel layer formed when these absorbent polymers swell in the presence of body fluids is more relevant than DUL values for determining the ability of these absorbent polymers to acquire and transport fluids, especially when the absorbent polymer is present at high concentrations in the absorbent structure. Porosity refers to the fractional volume that is not occupied by solid material. For a hydrogel layer formed entirely from a hydrogel-forming absorbent polymer, porosity is the fractional volume of the layer that is not occupied by hydrogel. For an absorbent structure containing the hydrogel, as well as other components, porosity is the fractional volume (also referred to as void volume) that is not occupied by the hydrogel, or other solid components (e.g., fibers).
Importantly, it has been found that hydrogel-forming absorbent polymers having higher porosities than those apparently desired by European patent application 532,002 are particularly suitable for absorbent structures having high concentrations of these absorbent polymers. (It is believed that hydrogel-forming absorbent polymers having DUL values below about 0.6 mm that are desired by European patent application 532,002 have relatively low porosities. It is also believed that hydrogel-forming absorbent polymers having relatively high porosities have DUL values above about 0.6 mm.) The openness or porosity of a hydrogel layer formed from a hydrogel-forming absorbent polymer can be defined in terms of Porosity of the Hydrogel Layer (PHL). A good example of a material having a very-high degree openness is an air-laid web of wood-pulp fibers. For example, the fractional degree of openness of an air-laid web of wood pulp fibers (e.g., having a density of 0.15 g/cc) is estimated to be 0.8-0.9, when wetted with body fluids under a confining pressure of 0.3 psi. By contrast, typical hydrogel-forming polymers such as Nalco 1180 (made by Nalco Chemical Co.) and L-761f (made by Nippon Shokubai Co., LTD) exhibit PHL values of about 0.1 or less
Moreover, it has been found that the PIE value of the hydrogel-forming absorbent polymer does not have to approach that of an air-laid web of wood pulp fibers in order to obtain substantial performance benefits when these absorbent polymers are present at high concentrations. These benefits include (1) increased void volume in the resultant hydrogel layer for acquiring and distributing fluid; and (2) increased total quantity of fluid absorbed by the absorbent polymer under demand wettability/gravimetric absorbency conditions (i.e., for the storage of fluid). Increased porosity can also provide additional performance benefits such as: (3) increased permeability of the resultant hydrogel layer for acquiring and distributing fluid; (4) improved wicking properties for the resultant hydrogel layer, such as wicking fluid upwardly against gravitational pressures or partitioning fluid away from an acquisition layer; and (5) improved swelling-rate properties for the resultant hydrogel layer to allow more-rapid storage of fluid.
Another important property at higher concentrations of these hydrogel-forming absorbent polymers is their permeability/flow conductivity. Permeability/flow conductivity can be defined in terms of their Saline Flow Conductivity (SFC) values. SFC measures the ability of a material to transport saline fluids, such as the ability of the hydrogel layer formed from the swollen hydrogel-forming absorbent polymer to transport body fluids. Typically, an air-laid web of pulp fibers (e.g., having a density of 0.15 g/cc) will exhibit an SFC value of about 200.times.10.sup.-7 cm.sup.3 sec/g. By contrast, typical hydrogel-forming absorbent polymers such as Aqualic L-74 (made by Nippon Shokubai Co., LTD) and Nalco-1180 (made by Nalco Chemical Co.) exhibit SFC values of at most 1.times.10.sup.-7 cm.sup.3 sec/g. Accordingly, it would be highly desirable to be able to use hydrogel-forming absorbent polymers that more closely approach an air-laid web of wood pulp fibers in terms of SFC.
Another factor that has to be considered in order to take full advantage of the porosity and permeability properties of the hydrogel layer formed from these absorbent polymers is the wet integrity of the region or regions in the absorbent member that comprise these polymers. For hydrogel-forming absorbent polymers having relatively high porosity and SFC values, it is extremely important that the region(s) in which polymers are present have good wet integrity. By "good wet integrity" is meant that the region or regions in the absorbent member having the high concentration of hydrogel-forming absorbent polymer have sufficient integrity in a dry, partially wet, and/or wetted state such that the physical continuity (and thus the capability of acquiring and transporting fluid into and through contiguous interstitial voids/capillaries) of the hydrogel formed upon swelling in the presence of body fluids is not substantially disrupted or altered, even when subjected to normal use conditions. During normal use, absorbent cores in absorbent articles are typically subjected to tensional and torsional forces of varying intensity and direction. These tensional and torsional forces include bunching in the crotch area, stretching and twisting forces as the person wearing the absorbent article walks, squats, bends, and the like. If wet integrity is inadequate, these tensional and torsional forces can potentially cause a substantial alteration and/or disruption in the physical continuity of the hydrogel such that its capability of acquiring and transporting fluids into and through the contiguous voids and capillaries is degraded, e.g., the hydrogel layer can be partially separated, fully separated, have gaps introduced, have areas that are significantly thinned, and/or broken up into a plurality of significantly smaller segments. Such alteration could minimize or completely negate any advantageous porosity and permeability/flow conductivity properties of the hydrogel-forming absorbent polymer.
Accordingly, it would be desirable to be able to provide an absorbent member comprising: (1) a region or regions having a relatively high concentration of hydrogel-forming absorbent polymer; (2) with relatively high porosities, and preferably permeability/flow conductivity properties more like an air-laid fibrous web; (3) that can readily acquire fluids from even high capillary suction acquisition layers under typical usage pressures; (4) in a matrix that provides sufficient wet integrity such that its capability for acquiring and transporting fluids is not substantially reduced or minimized, even when subjected to normal use forces. It would also be highly desirable to be able to use hydrogel-forming absorbent polymers in these absorbent members that, when swollen by body fluids, have higher PHL values such that: (a) the void volume per unit weight of absorbent polymer is closer to that of an air-laid fibrous web; (b) the demand wettability or gravimetric absorbency of the absorbent polymer under usage pressures is increased; and (c) the absorbent member preferably has increased permeability, improved wicking and/or improved swelling-rate properties.